1. Field of the Invention
The present invention relates in general to a Doppler-effect speed detecting apparatus adapted to detect or determine the ground speed of a motor vehicle, according to a Doppler effect or Doppler shift of waves transmitted and received by respective transmitter and receiver, and more particularly to techniques for improving the detecting accuracy of the apparatus.
2. Discussion of the Related Art
In a motor vehicle, the detection or determination of the running speed is necessary, for effecting an anti-lock or anti-skid control of braking forces applied to the vehicle wheels, for example. Conventionally, the vehicle running speed is estimated on the basis of rotating speeds of the wheels detected by appropriate wheel speed sensors. An example of such vehicle speed estimating means is disclosed in U.S. Pat. No. 4,778,223.
However, the accuracy of estimation of the vehicle speed on the basis of the detected wheel speeds is not satisfactorily high when the vehicle is running in a transient state, with brake application or rapid acceleration, for example. The vehicle speed estimating means utilizing the wheel speed sensors has another drawback. Namely, the actual running speed (ground speed) of the vehicle may deviate from the detected wheel speed due to a change in the diameter of the tire of the wheel during use of the vehicle.
In the light of the above drawback of the wheel speed sensors to determine the vehicle speed, there has been proposed the use of a Doppler-effect vehicle ground-speed detecting apparatus, which has (a) a transmitter for transmitting a wave (e.g., ultrasonic wave) toward the ground or road surface, (b) a receiver for receiving a portion of the transmitted wave reflected by the ground surface, and (c) a signal processor for determining the ground speed of the vehicle, according to a Doppler shift or Doppler effect, on the basis of the frequencies of the transmitted and received waves. An example of the apparatus of this type is disclosed in JP-A-2-287183 (published Nov. 27, 1990).
The Doppler-effect vehicle speed detecting or sensing apparatus utilizes transmitted and reflected waves which have components parallel to the running direction of the vehicle. The apparatus determines the running speed of the vehicle relative to the ground surface, on the basis of a Doppler shift of the frequency of the wave reflected by the ground surface with respect to that of the transmitted wave. The wave transmitter and receivers are attached to the body of the vehicle such that the boresight of the transmitter and receivers is angularly oriented relative to the vehicle running direction.
As described in the above-identified publication JP-A-2-287183, the apparatus is designed to determine the ground speed of the vehicle on the basis of the frequencies of the waves transmitted and received by the transmitter and receiver, on an assumption that the boresight or angle of the transmitter and receiver relative to the vehicle running direction is held constant. However, the boresight may differ on individual motor vehicles due to positioning error of the transmitter and receiver. Further, the boresight of the transmitter and receiver on a given vehicle may vary from time to time due to a change in the pitch angle of the vehicle, which arises from a change in the load distribution on the vehicle, which in turn takes place when the vehicle is decelerated with an abrupt brake applied thereto, for example. These varying or fluctuating factors as well as other adverse factors associated with the Doppler-effect speed detection lead to inaccuracy of the speed detection or determination by the apparatus during running of the vehicle in a transient state, with abrupt brake application or rapid acceleration, for example.
The Doppler-effect speed detecting apparatus as disclosed in the above-identified publication JP-A-2-287183 has signal generating means for generating a Doppler-frequency signal indicative of a Doppler frequency which is determined by the frequencies of the transmitted and reflected waves. This apparatus is adapted to determine the ground speed of the vehicle on the basis of the frequency of the Doppler-frequency signal, which is detected by counting the number of pulses of the Doppler-frequency signal per unit time.
In another type of Doppler-effect vehicle speed sensor proposed by the assignee of the present application, the vehicle ground speed is determined on the basis of the period of the Doppler-frequency signal, e.g., the period of each pulse of the signal, or the total period of a predetermined number of pulses of the signal. This type of apparatus is advantageous in terms of operating response, namely, assures relatively high response to a change in the vehicle speed. An example of the apparatus which uses the period of the Doppler-frequency signal to determine the vehicle speed is disclosed in co-pending application Ser. No. 07/942,776 filed Sep. 10, 1992, now U.S. Pat. No. 5,373,446, in which a reference range of the period of the Doppler-frequency signal is used to determine the vehicle speed. More specifically described, the reference range is determined from time to time by at least one previously obtained value of the period of the Doppler-frequency signal, such that the reference range has a constant width. If the presently detected value of the period is held within the determined reference range, the detected value is determined as the final value of the period. If the presently detected value is outside the reference range, the detected value is suitably adjusted or changed to obtain the final value. This adjustment is made on a concept that the deviation of the presently detected value of the period of the Doppler-frequency signal from the determined reference range was caused by external disturbances or noises associated with the detecting apparatus, and that some adjustment or compensation of the presently detected value is necessary to determine the final value.
However, the above type of the apparatus suffers from a problem when the actual vehicle speed is relatively low. Explained more particularly, the frequency of the Doppler-frequency signal decreases while the period of the signal increases, as the actual vehicle speed is lowered. On the other hand, the width of the reference range is kept constant irrespective of the actual vehicle speed. Consequently, when the actual vehicle speed is lowered down to 5 km/h, for example, the period of the Doppler-frequency signal is accordingly increased, and the presently detected value of the period may deviate from the reference range which is determined by the previously obtained value or values of the period, while in fact the presently detected value of the period is accurate. In this event, the presently detected value is unnecessarily and undesirably adjusted according to a predetermined rule to obtain the final value which is used to determine the vehicle speed. Thus, the apparatus is not capable of accurately detecting or determining the ground speed of the vehicle when the actual ground speed is relatively low.